In far eastern Poland in 2002 and 2003 strange planes landed on an old disused runway in a secluded forest - nine times.

The airport was closed but Mariola Przewlocka, the airport facilities manager, was told to accept the planes or "heads would roll".

Airport staff were told to stay away while the passengers were unloaded out of sight. Mini-vans with blacked-out windows drove them away to a former Soviet military intelligence base, where it is believed the CIA has its own zone.

Extraordinary rendition

Was Poland a staging point in the network of secret prisons established by the United States in their "extraordinary rendition" programme? Did these mystery flights bring al-Qaeda suspects to Poland?

"It didn't occur to anyone then that it might have something to do with transporting prisoners," says Mariola Przewlocka. "All the rigmarole surrounding the flights - now I think it may have been possible."

"Extraordinary rendition" is the CIA term for taking prisoners abroad for interrogation, a policy the US administration defends as a necessary tool in the "war on terror".

It denies that prisoners are taken to be tortured.

But it offers no explanation for transporting them around the world to countries that are known to use torture, such as Uzbekistan, Morocco, Egypt and Syria.

Torture denials

Binyam Mohammed claims to have been horribly tortured

Binyam Mohammed, a British resident from Ethiopia was "rendered" to Morocco in a Gulfstream N379P after he was arrested in Pakistan.

He says he was tortured there until he agreed to sign a statement his captors had prepared. The statement said that he was a member of al-Qaeda; that he had met Osama bin Laden and that he was part of a plot to explode a radioactive bomb in America.

His lawyer Clive Stafford Smith believes US denials on torture cannot be true.

"He was taken by the Americans to Morocco," he said. "He's not Moroccan so there's only one purpose and that's for him to go through a little bit of extraordinary interrogation."

Another rendition flight - this time a Boeing 737 - which stopped in Mallorca on its way to Afghanistan, was photographed by plane-spotters.

When human rights organisations, journalists, lawyers and plane-spotters compared notes, and when the dates were matched with flight logs and other prisoners' testimonies, the extent of the rendition programme began to be revealed.

It is alleged that the CIA flew their planes to 29 different countries and that there were 300 CIA landings in Europe alone, 80 in Britain.

How many prisoners were rendered is still not known. Nor is it known whether many were subjected to torture.

Damaging

European governments continue to deny they were involved.

Joseph Manchado, the plane-spotter who photographed the Boeing in Mallorca, is sceptical.

"I think the Spanish authorities knew that there were flights from Palma to Guantanamo. Clearly businessmen don't fly to Guantanamo. I don't suppose the authorities investigated - they just prefer to keep quiet and turn a blind eye to what was going on right under their noses".

Few people in America or Europe have cried for the fate of these Muslim men who may or may not be guilty, but who have never been tried or given a chance to defend themselves in court.

Some will think they deserve no mercy, in revenge for al-Qaeda's crimes committed in their religion's name.

But the former head of the CIA in Europe Tyler Drumheller admits the policy has been damaging. In an exclusive interview with the BBC he says: "It's a mess, and it's going to get worse. A lot of things were done after 9/11 that are going to be looked at for years to come. There are going to be commissions, inquiries, court cases."

Mr Drumheller was head of clandestine operations between 2001 and his resignation in 2005.

He cannot speak of the secret prisons, for fear of prosecution himself, but he believes the buck stops with the US President.

Switzerland: Criminal probe into use of Swiss airspace to fly kidnapped imam from Italy to Germany

By September 2006, the White House was forced to admit that "a small number of suspected terrorist leaders and operatives captured during the war had been held and questioned outside the United States in a separate programme operated by the Central Intelligence Agency".

But the president would not say where detainees had been held, or divulge details of their confinement.

Slowly Europe's democracies are cranking into action in a belated attempt to hold their own governments to account.

In Italy, a former chief of military intelligence is in court. Warrants are out for the arrest of CIA agents formerly based there and the government is being accused of a cover-up.

The former President of Poland, Aleksander Kwasniewski, and the former Polish Defence Minister, Radoslaw Sirkorski, have denied that Poland and the military airbase in question were involved.

The UK Government has said it does not know and has no way of finding out who was aboard the 80 CIA flights that landed on British soil.

This World: "Mystery Flights" will be broadcast on Thursday 24 May 2007 at 2100 BST on BBC Two.